An image processing apparatus detects an edge of a black character in a document image, and performs an emphasis process for the edge (see PATENT LITERATURE #1).
Further, another image processing apparatus detects a black edge in a document image, replaces CMY (Cyan, Magenta, Yellow) values of a pixel having the lowest density among adjacent pixels to a pixel on the black edge with CMY values of this pixel on the black edge, and increases a black value of this pixel on the black edge (see PATENT LITERATURE #2).